1. Field of the Invention
The present invention is employed in optical communication which sets up an optical path, and then sets up a connection over this optical path. In particular, the present invention relates to a technique for selecting a wavelength for setting up an optical path, and to an optical path setup technique for a high capacity optical path network which is implemented with an optical path switching device (for example a photonic router).
2. Description of the Related Art
The source node described in this specification is a node which transmits a path setup request, while the destination node is a node to which the path setup request is addressed. Furthermore, the optical path which is set up may either be a unidirectional path or a bidirectional path. In the case of a bidirectional path, it is possible for either the source node or the destination node to operate as a source for transmitting data or as a data recipient.
Due to the vast increase in data communication traffic upon the internet and the like, currently rapid progress is occurring with the introduction of node devices which offer a throughput of several Tbit/sec at the present time, and of 10 to 100 Tbit/sec in the near future. Photonic routers are a powerful means for implementing node devices which are endowed with this order of large scale transmission capability; for example, refer to the publication by K. Shimano, A. Imaoka, Y. Takigawa, and K. Sato in Technical Digest of NFOEC' 2001, vol. 1, pp. 5-13, July 2001. An overall view of an optical path network which employs such photonic routers is shown in FIG. 19. With these devices, the management of the optical path network is performed by each node in a distributed manner, and the setting up of the optical path connections is performed based upon signaling procedures between the various nodes.
The conventional technique employs the following type of procedures for setting up a path over this type of optical path network from the source node to the destination node.
First, when the source node transmits a path setup request towards the destination node, this path setup request is forwarded in order by each node upon the route which must be set up a path, and arrives at the destination node. After this, when the destination node transmits a resource reservation request towards the source node for reserving the resources (for example the wavelengths) which are to be used for setting up the path, this resource reservation request is forwarded in order by each node upon the above described route in the reverse direction to the direction when the path setup request was forwarded, and arrives at the source node. At this time, along with the resource reservation request being forwarded in order from the destination node to the source node, each node upon the above route performs reservation of the appropriate resources.
However, due to the influence of other connections, it may happen that the situations with regard to available resources such as wavelengths and the like at the time point at which each node upon the above described route receives the path setup request, and at the time point at which it receives the resource reservation request, are different. Due to this, despite it was possible to reserve the available resources when the path setup request was received, the situation may also undesirably come to pass that, when the resource reservation request has been received, it becomes impossible to reserve the available resources. Accordingly, there is a problem with the conventional procedure in terms of poor certainty and reliability in setting up a path.
On the other hand, a key point for implementing reduction in cost of the type of optical path network described above is to reduce to the utmost extent possible the number of wavelength converters which are provided within the network. As a means to this end, it is effective to prepare the wavelength converters optionally, and to set up the optical path while employing as few wavelength converters as possible.
This is because wavelength conversion at the present time is extremely expensive, so that, if it can be arranged for wavelength conversion not to be required, it is possible to perform transparent transmission of the signal while rendering electrical conversion and the like completely unnecessary.
Here, FIG. 20 shows an operation of conventional wavelength selection. Wavelength selection is performed link-by-link using the RSVP (Resource Reservation Protocol) protocol for signaling. Among the available wavelengths λ1 through λ4, the node A is connected to the node B using the wavelength λ1, which is chosen randomly or because it is the lowest numbered wavelength available. At the node B, the same wavelength λ1 is checked with the aim of performing as little wavelength conversion as possible, and, if the wavelength λ1 is available as it happens, the node B is connected to the node C using this wavelength λ1.
On the other hand, since, at the node C, the available wavelengths are λ3 through λ5, wavelength conversion will unfortunately be required whichever of these wavelengths is selected. As a result, the wavelength relay situation becomes λ1->λ1->λ3, and wavelength conversion at the node C becomes necessary.
In the example shown in FIG. 20, if it had been possible to set up the entire connection with the wavelength λ3, the wavelength relay situation would have become λ3->λ3->λ3, so that no wavelength conversion at all would have been required. However since, conventionally, there was no means for selecting the available wavelength globally, accordingly, in the conventional wavelength selection procedure, it has not been possible to perform this type of consideration in advance, in order to select the available wavelength globally.
Furthermore, for implementing the setting up of an optical path while restricting the number of wavelength converters which are employed, it is necessary to optimize the selection of the wavelength channels between the source node and the destination node. For example, as shown in FIG. 21, for setting up an optical path from the node #1 to the node #5, the optical channel which is shown by the broken line is an available optical channel, and the path λ1->λ3->λ3->λ1 is that optical path on which the number of wavelength conversions at the transit nodes is the least.
However, the signaling procedure in such a conventional optical path setup scheme does not have the means for implementing the wavelength conversion and wavelength channel selection along the intermediate route while taking account of the wavelength conversion cost at each node as described above.